1. Field of the Invention
The present invention relates to ambient air vaporizers and heaters and more particularly to unidirectional flow vaporizers and heaters for cryogenic fluids.
2. Brief Description of the Prior Art
Atmospheric vaporizers have been known in the art for a long time. These devices are used in industry to vaporize relatively large quantities of a cryogenic liquid which is needed in the gaseous form for certain manufacturing and other operations. In essence, an atmospheric vaporizer is a heat exchanger which utilizes ambient heat to vaporize a very low boiling (cryogenic) liquid. A typical example for the use of atmospheric vaporizers/heat exchangers is the vaporization of liquid oxygen for use in industrial processes and welding operations.
State of the art atmospheric vaporizers/heat exchangers include a plurality of heat exchange elements which are finned tubes made of good heat conducting materials (usually aluminum). The finned tubes are mechanically assembled to one another and to a substantially rigid frame. Flow of the cryogenic fluid through the tubes is generally in a serial fashion i.e. from one tube to another to maintain the height of the vaporizer/heat exchanger within reasonable limits, for example, of the order of 6 to 25 feet. The relatively large surface of the fins facilitates efficient heat exchange with the environment; in other words, the fins promote relatively efficient absorption of heat from the ambient atmosphere and thus provide the heat required for vaporization and heating of the cryogenic liquid.
The prior art serial vaporizers connect a number of vertically oriented heat exchange tubes in series to vaporize the cryogenic liquid which enters the heat exchanger at a temperature well below the freezing point of water, e.g. about -300.degree. F., and heat it to a temperature suitable for manufacturing or other operations e.g. within 10.degree. to 40.degree. F. of the ambient temperature.
A serial atmospheric vaporizer/heat exchanger, including specific dimensions for the external and internal fins of its elongated finned tubes, is described in U.S. Pat. No. 4,479,359.
To obtain the desired flow rate several groups of serially connected tubes may be placed in parallel. One prior art vaporizer manufactured by the assignee of this application utilizes 36 heat exchange tubes with each group of 6 connected in series. With such an arrangement, the flow of the cryogenic fluid is bi-directional i.e. up through one tube, down through the next etc. The temperature of the cryogenic fluid will increase from its entrance temperature e.g. about -300.degree. F. to its exit temperature e.g. 20.degree.-60.degree. F., as it travels through the serially connected tubes.
Since the accumulation of ice on the external fins is a function of the interior temperature of the tube, the largest portion of the ice packs will form on the first several tubes. As a result large ice packs typically will accumulate at both the bottom and top of the first two, three or even four tubes in a six tube array with little, if any, ice accumulating on the tubes near the outlet unless the ambient temperature is near or below freezing. The iced unit will therefore have an uneven distribution of ice over the tubes and a relatively high center of gravity. The high center of gravity is particularly troublesome in meeting stringent design code requirements, e.g. 100 miles per hour wind loads and seismic zone 4 safety criteria, because of the bracing required.
In addition to the high center of gravity problem, the difference in temperature between the horizontal sections of adjacent heat exchange tubes in series with one another results in differential thermal gradients between the tubes and the need to mechanically compensate for such gradients in the lateral plane. Also, the most efficient heat transfer occurs in only one-half of the prior art series connected elements e.g. counter-flow between the downward flow of ambient air by natural convection and the upward flow of the cryogenic fluid The remaining elements are in co-current flow pattern with the downward flow of fluid and downward flow of air.
There is a need for a more efficient ambient air vaporizer with full counterflow which provides an improved ice build-up pattern with an attendant low center of gravity and an improved structural integrity with respect to thermal expansion and contraction.